Variable speed transmissions have been designed and built for well over one hundred years, but the basic principles of efficient and long life operations have been largely ignored. It has been occasionally recognized that for two bodies to transmit a rolling force between them the two bodies must both be cones with their apexes at one identical point. The cones can have many different shapes. I am assuming that the cones of interest here are fight angle, circular cones. This means that if a plane is passed at a right angle to the axis of the cone the intersection will be a circle. This distinguishes the circular cone from elliptical or other non-circular cones.
The cone can be a "male" cone of gradually larger and larger angle between opposite elements, finally becoming a fiat disc. A fiat disc is a cone of 180 degrees. The cone then can become a "female" cone of progressively smaller inside angle.
A cylinder is a cone with an included angle of 0 degrees, with the apex at infinity. This is why two cylinders can roll on each other as can any two other straight line cones. A cone can also roll on a flat disc as long as its apex is at the center of the disc.
A common version of a variable drive has been a cylinder which is rolling on a flat disc. While the disc is a cone of 180 degrees included angle, the wheel is not a proper cone and the resulting contact between the two parts does result in a driving relationship but also in what is called "spin" at the small area of contact. This spin leads to waste of energy, the production of heat and large increase of wear.
Many prior versions of variable speed friction drives use discs, cones or toruses where the torque is transmitted from one such element to an intermediate member, called an idler, and from the idler to another element that may also be a cone, or a disc or a torus. Such drives suffer from the further disadvantage in that the efficiency is further reduced. If the efficiency of one friction point is, say, 90%, and the second point is also 90%, the over-all efficiency is the product, or 81%.
A third fault found in prior art is that the cones or discs of the contacting elements are curved so as to increase the area of contact between them. This prevents the elements from correctly rolling on each other and causes unnecessary friction very much like spin and with the same deleterious effect.
The above difficulties with prior art devices does not mean that many of them are not useful for low power, instrument or control use. But for high power application such as automobile transmissions or for large machinery use, the friction devices must make use of the best possible technology.
In the book "Continuously Variable Transmissions for Passenger Cars," published by the SAE, there is a reproduction of a drawing that appeared in a NASA Technical Memorandum No. 83397, September 1983, P. 15. It is interesting to note that not one design in this collection of 15 variable speed friction drive mechanisms meets the criteria of two cones, or parts of cones that roll on each other and that have a common point for their apexes. Several of them use idlers between their main rotating elements, further lowering their efficiencies. In some cases, the idlers take the form of belts or rings, but the effect on efficiencies is the same.
In searching through the art of variable speed friction drives I found three inventions that are worth special comments.
The first is U.S. Pat. No. 5,014,565 issued to Stephenson. He fully realizes that the rolling wheel 12 in his FIG. 1 must have its axis meet the axis YY of the wheel 10 at a common point. The surface of the wheel 10, however, is concave and that prevents true rolling between the wheel 12 and the wheel 10. Stephenson states in the bottom paragraph of column 4 that "a pure rolling contact exists" between element 10 and wheel 12 at any point "on their respective contact faces." This is correct if the elements touch at only one point. If the area of contact is increased by elastic deformation or by the use of a suitable intermediate fluid, the rolling is not pure and some sliding will be present. There is a statement at the top of column 5 that reads "Widening the contact area at point C does not give rise to frictional losses, unlike the prior art transmission described earlier." This is not correct. Widening the contact area will give rise to frictional losses. The use of curved surfaces increases the contact areas but this expedient also gives rise to friction losses.
To clarify this point it should be noted that two cones with a common apex point can roll on each other even if they were to contact along their entire length. This cannot be done with a concave cone in full length contact with another element that is curved to fit. The arrangement of FIG. 1 of this patent also requires that the plate 10 be lowered or raised as the wheel 12 moves in or out on its axis XX. The plate can be kept at a constant height but the wheel axis must be raised and lowered as the speed ratio is changed.
U.S. Pat. No. 3,158,041 to Rae discusses the problem of rolling a simple conical wheel on a surface of a straight sided cone (FIG. 1). Rae correctly points out that such an arrangement can be correct at only one position of the wheel and will cause spin at any other position. Rae also points out that using a cylindical wheel would be even worse. At no point will be rolling be correct.
To improve the situation Rae describes a cone whose surface is such that a tangent (in the plane of the axis) from any point on the surface to the axis of the cone will have a fixed length (FIG. 2). A conical roller having this identical length of axis, will roll well on the surface of the cone. Since the roller has appreciable thickness the rolling will not be completely correct. One edge of the roller will contact one tangent to the cone surface while the other edge will contact another tangent. The two tangents of the curved cone cross the axis at two different points while the axis of both edges of the conical roller is of only one fixed length. This will prevent pure rolling but the effect will be small if the roller is thin. The use of a thin roller on a curved cone leads to the practical difficulty in that the roller cannot remain parallel to itself as it is made to contact various positions on the curved cone. This is why the mechanisms shown in FIG. 3 are so unusual and so complex.
This patent also shows a differential gear device by which changes of speed by a friction device can produce wide changes of speed, including zero speed. Such devices have nothing to do with the subject of friction devices, but it should be noted that their efficiencies are zero at zero speed and very low at low speeds. They illustrate poor engineering design and should not be pan of an otherwise interesting patent.
A difficulty that is also present in this patent is that the friction contact between the cone and the roller make use of two male elements. The area of contact is therefore very small and this required very large contact forces to produce reasonable friction torques.
The French Patent No. 1,003,009 to Honore issued in 1951 is the most interesting of the three that attempt the use rolling cones whose axes meet at a common point. Honore uses a male cone that rolls on a section of a female cone with the contact between them being at a point. As long as the area of contact is very small, this results in good rolling action. If the contact area is increased by using two surfaces whose curves match for an appreciable distance, then pure rotation cannot occur.
I covered this point in my discussion of the two prior patents. There is one feature of the Honore patent that is of special interest to me. This is shown in FIG. 1 where the male cone 1 is shown (in dotted lines) fully engaged with the female ring 2 so that there is no more rolling contact between them, but the two now constitute a cone-clutch. To make this clutching possible, the dimensions of the male cones and the female tings (portions of cones) must not be too different in radial dimensions. This, in turn, limits the practical speed ratio that are easy to obtain with his designs. To overcome this difficulty, Honore also resorts to the use of a differential gear in FIG. 4. As I stated before, this is most inappropriate method of obtaining low speeds and is not worthy of the technology of this patent.
The embodiments of the invention covered by Patent No. 1,003,009 all require that the relative motions of the male and female members consist of motions along the axes of the elements and also changes of angles between the axes so that the instantaneous apexes of the cone elements in contact come together at common points.
Honore does not illustrate how such motions are produced except in FIG. 9 where curved guide surfaces are used to move the rings 2' and 2" as they contact the male cones. The use of gears that do not mesh properly, as in FIGS. 4, 5, 8, 9, and 10, is mentioned only in passing as, for example, in gears 11 and 12 of FIG. 4. This is also true of the meshing of the cylindical gear 23 with the gear 22 of the cone ring 2. The gears of FIG. 8 and 9 are stated to permit the complex motions of the rings to which they are attached.
Summarizing my comments of the prior art, I believe that all of the designs that have been and are now built commercially suffer from inefficiencies and wear due to improper rolling action. No successful automobile variable speed friction drive of the type of interest here has emerged. Variable speed belt drives, hydraulic drives and other mechanisms have been used successfully, but they have no bearing on my invention.